Satellites and other vehicles are in widespread use for various purposes including scientific research and communications. Many scientific and communications missions, however, cannot be accurately fulfilled without consistently monitoring and controlling the 3-axis attitude and angular velocity of the vehicle. Attitude may be described as the vehicle orientation with respect to some frame, for example, the Earth-Centered Inertial (ECI) frame. In many applications, the vehicle must be oriented to transmit signals in particular directions or to receive signals from specifically located sources. Furthermore, in such a situation, the vehicle angular velocity must be such so as to maintain the desired orientation, over time. Without accurate control over vehicle 3-axis attitude and angular velocity, the transmission or reception of such signals is hindered and at times impossible.
As is described in U.S. Pat. No. 6,285,927, which patent is hereby incorporated by reference, optical sensors such as star trackers are commonly used to assist in the navigation and control of spacecraft. Typically, star trackers report the position of an observed star in a two-dimensional star tracker reference frame, for example, indicating the star position in terms of a horizontal and vertical displacement (H and V, respectively) from the boresight of the star tracker. Such measurements are obtained with charge coupled device (CCD) or similar arrays that collect incident radiation (e.g. photons) over an integration period. The star trackers typically report the centroid of the measurements taken over the integration period and the center time of the integration. Reported star tracker positions are sufficiently accurate if the attitude of the spacecraft remains reasonably constant over time, as the centroid of the apparent motion of the stars over the integration time will be close to the actual position of the star. Star tracker measurements are also reasonably accurate when the spacecraft experiences a constant angular velocity ω over the integration time, as the centroid of the apparent motion of the star over the integration time will also be close to the actual position of the star at the center-time of the integration.
However, the measured centroid is shifted from the true position of the star at the center time of the integration when the apparent velocity of stars crossing the star tracker field of view changes during the integration period. Star trackers can meet angular accuracy requirements if the angular acceleration of the star tracker small (e.g. less than 0.1 deg/sec/sec.), but for accelerations above that, the star tracker data includes a bias error that corrupts the spacecraft attitude estimate.
U.S. application Ser. No. 10/360,020, to the same assignee, which is incorporated by reference herein, addresses a technique for correcting reported star positions or the estimated star positions so that the star motion induced position errors can be compensated for. The technique is used for star motion that is subject to a constant angular acceleration crossing the CCD arrays during star integration time. When the acceleration varies with time, as with vibration, the correction may be incomplete.
The issue of vibration on star sensor processing is discussed in U.S. Pat. No. 5,012,081. The motion compensation solutions discussed there, however, have fairly burdensome hardware requirements.
What is needed is a system and method for improving the accuracy of optical sensor data. The present invention satisfies that need for a wide variety of vehicles and optical sensors, and is particularly useful for star tracker data used to control agile spacecraft having tight pointing requirements.